I'll play a little......
I'd go to 1775 and introduce the Minie ball and rifled musket.
It'd still be a flint lock, but it would outclass anything the Brits had.
1939 I'd convince the military to adopt a modified AK-47 chambered in an American version of the 7.62x39 only in a .30 caliber.
In 1915 I'd kick Army and Marine butts and force the start of a well trained, dedicated scout/sniper program, and I'd equip them with a true sniper rifle.
After 1918 it would be continued for use in WWII/Korea.
We would not have started WWII with the Brewster Buffalo "fighter", and the P-40 Warhawk would be getting removed from service in favor of the P-51 Mustang.
By mid 1944 the F-86 Sabre jet would be over Berlin.
The M26 Pershing tank would have been at Kasserine Pass and Patton would be in command.
When Hitler took back the Rhineland in 1936 I would have sent two Texas Rangers to run him off, and stopped WWII.
